In recent times, reducing the volatile organic content (VOC) of coating formulations has been a major research focus in the paint and coatings industries. This is mainly driven by government regulations to reduce volatile organics released into the atmosphere. To meet this challenge, higher solid, waterborne and powder coatings have received increased attention as potential substitutes for solvent based coatings system. However, most of these coating systems are less resistant to chemical and solvent attack, they are lower in gloss and, in many cases, they are substantially more expensive than solvent based systems.
Polyurethanes have been particularly important in coatings by virtue of their outstanding properties. Waterborne polyurethanes and their use for coatings have been known for many years. They possess many properties that are significantly superior to other waterborne systems, but they are still insufficient to match the performance of solvent based two-component polyurethane systems. This is especially true regarding hardness, gloss, and solvent resistance. The development of two-component waterborne polyurethanes has helped overcome some of these deficiencies. However, most of the two-component waterborne systems are not completely free of VOC and are expensive.
Representative examples from the patent literature which describe advancements in two-component water dispersible polyurethanes follows.
U.S. Pat. No. 5,075,370 (Kubitza et al., 1991) discloses a two-component, aqueous coating composition which contains as binder components a polymer polyol component having carboxylic or sulfonic acid groups, such as a hydroxyacrylate polymer, and a polyisocyanate component in quantities corresponding to an NCO/OH equivalent ratio of about 0.5:1 to 5:1. The polyisocyanate component is emulsified in an aqueous solution of the polymer polyol component.
U.S. Pat. No. 5,194,487 (Jacobs, 1993) discloses two-component, aqueous polyurethane dispersions for coatings which contain an isocyanate-reactive component in which there is an aqueous dispersion of a hydroxy functional polyurethane and a polyol, and a water dispersible polyisocyanate component. The two components are present in an amount to provide an equivalent ratio of isocyanate to hydroxyl groups of 0.8:1 to 4:1. Organic solvents can be used in the production of the hydroxy functional polyurethanes and in reducing the viscosity of the water dispersible polyisocyanate component.
U.S. Pat. No. 5,331,039 (Blum et al., 1994) discloses two-component coating compositions in which the aqueous binder composition is based on at least two water dispersible polyol polymers and a polyisocyanate component dispersed therein and the NCO:OH equivalent ratio is 0.2:1 to 5:1. Five general groups of water dispersible polyol polymers are described; for example, polyester resin containing urethane, carboxyl and/or hydroxy groups, polyacrylate resins containing hydroxyl, carboxylate and/or sulfonate groups, and an acrylate-grafted polyester resins containing hydroxyl, carboxylate and/or sulfonate groups.
U.S. Pat. No. 5,552,477 (Dhein et al., 1996) discloses a method for producing an aqueous coating agent based on a polyol component dissolved and/or dispersed in water which is combined with a reactive diluent having at least one group which is reactive with isocyanate groups, and a polyisocyanate component emulsified in the aqueous solution or dispersion of the polyol. The polyol component consists of at least one water-dilutable polyacrylic resin or polyester resin containing hydroxyl groups and chemically bonded carboxylate and/or sulphonate groups, and the polyisocyanate component is not hydrophilicly modified. The equivalent ratio of isocyanate groups to groups capable of reacting with isocyanate groups is 0.5:1 to 5:1.
U.S. Pat. No. 5,508,340 (Hart, 1996) discloses a substantially solvent free aqueous two-component polyurethane coating composition in which one component is a combined polyol and amine phase and the other component is a polyisocyanate. The ratio between the isocyanate equivalents and the sum of the active hydrogen equivalents of the polyols and amines is at least 0.5:1.
The following publications provide background on the way high Tg polymer particles affect latex films:
J. Feng, et al., "Effect of Hard Polymer Filler Particles on Polymer Diffusion in Low-Tg Latex Film," Macromolecules, Vol.31, 1998, pages 5290-5299.
S. T. Eckersley, et al., "Mechanistic Considerations of Particle Size Effects on Film Properties of Hard/Soft Latex Blends," Journal of Coatings Technology, Vol. 69, No. 864, January 1997, pages 97-107.